The invention relates to solar-radiation collection apparatus of the self-tracking high- concentration variety, high-concentration being understood to apply to concentrations exceeding 100 suns, and especially in the range of 500 or more suns.
Our U.S. Pat. No. 4,086,485 discloses such a system involving multiple use of self-tracking collecting optical systems, each with its associated utilization device, such as a photovoltaic cell. The concentration of solar radiation is high and, for efficient utilization, the full image of the sun covers the exposure surface substantially only when the axis of the optical system is aligned with the sun. The angle subtended by observed diametral limits of the sun thus represents substantially the limit of optical-system axial misalignment which can be tolerated by the self-tracking mechanism. As a practical matter, the system is inherently self-tracking as long as a portion of the sun's image remains on the cell, since clock-operated diurnal-drive mechanism enables approximately correct optical-axis orientation for intervals in which the sun is not available, i.e., when the sun cannot be sensed for tracking or any other purpose. But for more prolonged intervals, for example days without sunshine, the inherently small off-axis angle of self-tracking capability can present a problem.
Further, the collecting optical system of said patent is of the Cassegrain variety, involving a centrally apertured primary mirror and a mounting of the cell to receive secondary-mirror reflected light via the aperture and at a location offset behind the primary mirror. And since the axis of the optical system must at all times be inclined, consistent with the sun's seasonal and diurnal elevation ranges, there is a substantial exposure of the cell to vertical convection currents of air, with attendant loss of thermal efficiency.